1. Field of the Invention
This invention relates to monolithic amplifiers suitable primarily for handling microwave or radio-frequency (RF) signals. In particular, the invention relates to the design of bipolar transistor microwave/RF amplifiers that are resistant to severe load mismatch and/or high overdrive conditions.
2. Description of Related Art
Wireless handset power amplifiers often include one or more heterojunction bipolar transistors (HBTs) that provide efficient amplification at the high frequencies of present wireless systems. HBTs generally comprise several smaller HBTs connected in parallel. The smaller HBTs, also referred to as cells, may be identical to each other but may also differ to the other cells in the HBT depending on design considerations. Generally, HBTs are preferred over bipolar junction transistors (BJTs) because of the higher gain, higher breakdown voltage, and higher saturation velocity of the HBT. GaAs HBTs are preferred over silicon, despite their greater cost, because the high electron mobility in GaAs enables GaAs HBTs to operate at the gigahertz frequencies of our present wireless systems. HBTs, however, may fail from thermal runaway brought on by a severe load mismatch and/or high overdrive condition. The Wireless GSM standard requires that the amplification stage survive a 10:1 Voltage Standing Wave Ratio (VSWR) mismatched load at all phases under full RF drive and high collector voltage, which is normally higher than 4.5 V. Under such conditions, the load line is distorted and there are significant increases in the collector and base currents through the HBT. The large collector and base currents cause self-heating in the HBT and increase the dissipated power. If the dissipated power exceeds a threshold, the HBT undergoes thermal runaway and is irreversibly damaged.
FIG. 1 illustrates a typical HBT amplification stage where the base 120 of the HBT 125 is biased with constant voltage at 110, VIN, through a lumped resistor 112, R1, and a distributed ballast resistor 114, R2. As used hereinafter, a distributed resistor is a resistor that is electrically connected to each cell comprising the HBT. Input RF power at terminal 105, RFin, is supplied through blocking capacitor 107 separating the DC and RF input lines. An additional distributed resistor 116, R3, is placed in the RF path of the base for stability.
Collector current or voltage clipping circuits are added to the circuit shown in FIG. 1 to limit the collector and base currents through the HBT. Such circuits are usually implemented in silicon complementary metal oxide semiconductor (Si-CMOS) because of cost considerations. Such a design requires a combination of GaAs HBT with a Si CMOS or other hybrid approaches. These hybrid approaches result in higher manufacturing costs and may even place a lower limit on possible device sizes. Therefore, there remains a need for monolithic RF/microwave power amplifiers that are capable of surviving severe load mismatch or overdrive conditions.